The present invention relates to mechanical clamps for clamping a semiconductor wafer on a platen for processing of the semiconductor wafer. More particularly, the invention relates to a device for automatically centering a wafer on a platen spider before clamping the wafer on a platen in a medium current implanter for processing the wafer.
As semiconductor wafers processed in the semiconductor production industry progressively become larger in diameter, batch-processing ion implanters for processing multiple semiconductor wafers at one time are being replaced by ion implanters which process one semiconductor wafer at a time. Such single-substrate-processing ion implanters have two multi-articulated robot arm assemblies (substrate holding robots or substrate holding devices) with respect to one ion beam. These articulated robot arm assemblies allow the ion implanters to operate efficiently. The articulated robot arm assembly moves the semiconductor wafer in a direction perpendicular to the ion beam to allow the ion beam to be applied to the substrate. The robot arm assembly also replaces the semiconductor wafer with a new wafer.
The single-substrate processing ion implanter typically further includes two load-lock chambers into which are placed cassettes storing multiple semiconductor wafers. Feed robots for transferring the wafers are positioned between the load-lock chambers and the articulated robot arm assemblies, thus defining two parallel feed lines. Wafers are reciprocally transferred by the feed robots along the feed lines between the cassettes and the articulated robot assemblies.
Because the substrate holding devices and the cassettes are provided exclusively for respective groups of wafers, movement of the wafers is limited and cannot be flexibly adapted to various operating conditions. For example, if two successive substrates are made defective by an ion implantation failure due to a malfunction of an ion source or the like, then the subsequent processing is adversely affected because the defective substrates must be returned to the respective cassettes.
It is advantageous that each of the feed robots disposed between the load-lock chambers and the articulated robot arm assemblies has a clamp machanism for reliably and quickly feeding a wafer. One type of clamp mechanism utilizes a vacuum force, which cannot be used in a vacuum atmosphere or ambient and tends to apply a large quantity of particles to a wafer as it contacts a wide area of the wafer. Another clamp mechanism has an actuator for opening and closing clamp fingers.
Each of the articulated robot arm assemblies includes a substrate holding mechanism which includes a wafer holder or platen for holding the wafer, a clamp mechanism for mechanically securing the substrate on the platen, a substrate attracting mechanism such as an electrostatic chuck for attracting the substrate under electrostatic forces, a substrate cooling mechanism for cooling the substrate, and a substrate attaching and removing mechanism including a clamp operating mechanism for operating the clamp mechanism. These elements are provided on the end of the assembly.
Referring to FIGS. 1 and 2 of the drawings, a typical conventional substrate holding mechanism, such as that used in a Varian EHP500 medium current implanter, is generally indicated by reference numeral 10 and includes a platen 12 fitted with a mechanically-actuated platen spider 14 having an extendible spider pillar 16. The spider pillar 16 extends through a central platen opening (not illustrated) and is engaged by a spider actuation mechanism (not illustrated) for selectively raising and lowering the spider pillar 16. Accordingly, the platen spider 14 is operable between an upwardly-extended position, illustrated in FIG. 1, for receiving a semiconductor wafer 20 from or transferring the wafer 20 to a robot wafer handler (not illustrated), and a lower position (not illustrated) for clamping of the wafer 20 to the platen 12 and processing of the wafer 20. When the platen spider 14 is in the lower position on the platen 12, a robotic arm (not illustrated) inserts a mechanical clamp 18 around the wafer 20 to secure the wafer 20 to the platen 12 for implanting of ions into the wafer 20. Accordingly, as illustrated in FIG. 2, the mechanical clamp 18 includes multiple, inwardly-extending clamp fingers 19 which contact the peripheral edge of the wafer 20.
One of the problems frequently encountered in the conventional substrate holding mechanism 10 is that the wafer 20 is not accurately centered on the platen spider 14 when the mechanical clamp 18 is fitted thereon. Consequently, as illustrated in FIG. 2, some of the clamp fingers 19 tend to engage one edge of the wafer 20 more tightly than the other clamp fingers 19 engage the opposite edge of the wafer 20. This can cause micro-fractures to form in the wafer 20 where the clamp fingers 19 more tightly engage the wafer 20, thus resulting in significant wafer scrapping as well as machine down time due to the damage to the wafers 20.
Accordingly, a device is needed for automatically centering the wafer 20 on the spider 14 prior to inserting the mechanical clamp 18 on the wafer 20, to facilitate equal pressure distribution of the clamp fingers 19 against the edge of the wafer 20 and thus, prevent excessive pressure of any of the clamp fingers 19 against the wafer 20 and micro-cracking or fracturing of the wafer 20.
An object of the present invention is to provide a device for automatically centering a semiconductor wafer on a platen for processing of the wafer.
Another object of the present invention is to provide a device for preventing micro-cracking or fracturing of a semiconductor wafer in an ion implanter upon clamping of the wafer to a platen.
Still another object of the present invention is to provide a device for centering a semiconductor wafer on a platen spider to facilitate substantially uniform contact of a mechanical clamp with the edges of the wafer upon fitting the clamp on the wafer and securing the wafer to the platen.
Yet another object of the present invention is to provide an auto-centering device which is adaptable to the Varian EHP500 medium current implanter for automatically centering a semiconductor wafer on a platen spider as the wafer is lowered on the platen spider from a wafer loading and unloading position to a wafer processing position.
In accordance with these and other objects and advantages, the present invention comprises a substrate holding mechanism which is particularly adaptable to automatically centering a semiconductor wafer on a platen spider as the wafer is lowered from a wafer loading and unloading position to a wafer processing position in a medium current implanter such as a Varian EHP500. Upon subsequent placement of a mechanical clamp on the wafer to hold the wafer on the platen, the clamp fingers of the clamp engage the edge of the wafer with substantially uniform pressure to prevent micro-cracking or fracturing of the wafer.